The present invention relates to a chemical or bath for surface-treating aluminum or its alloy, and more particularly to a surface treatment chemical or bath suitable for the surface treatment of aluminum cans for drinks.
Aluminum and its alloy are conventionally subjected to a chemical treatment to provide them with corrosion resistance and to form undercoating layers thereon. A typical example of such chemical treatment is a treatment with a solution containing chromic acid, phosphoric acid and hydrofluoric acid. This method can provide a coating having high resistance to blackening by boiling water and high adhesion to a polymer coating film formed thereon. However, since the solution contains chromium [VI], it is hazardous to health and also causes problems of waste water treatment. Thus, various surface treatment solutions containing no chromium [VI] have already been developed.
For instance, Japanese Patent Laid-Open No. 48-27935 discloses a method of treating aluminum or its alloy with a solution of pH of 3-5 which contains a water-soluble zinc salt, a water-soluble vanadate, a water-soluble fluoride or fluorine complex salt, an oxyacid salt of halogen as an oxidizing agent, etc. Japanese Patent Laid-Open No. 55-131176 discloses a method of surface-treating a metal [particularly aluminum] with a phosphate treating solution of pH 1.5-3.0 containing vanadate ion. Japanese Patent Publication No. 56-33468 discloses a coating solution for the surface treatment of aluminum, which contains zirconium, phosphate and an effective fluoride and has pH of 1.5-4.0. Further, Japanese Patent Laid-Open No. 56-136978 discloses a chemical treatment solution for aluminum or its alloy containing a vanadium compound, and a zirconium compound or a silicon fluoride compound.
However, in the method disclosed in Japanese Patent Laid-Open No. 48-27935, treating time is as long as 3-10 minutes, meaning poor efficiency, and the formed coating layer is turned gray, unsuitable for aluminum cans for drinks. Further, the conversion coating produced by this method does not have sufficient adhesion to a polymer coating film of paint, ink, lacquer, etc.
With respect to the method disclosed in Japanese Patent Laid-Open No. 55-131176, since it is a non-rinse method, it is not applicable to cans for drinks. In addition, the formed conversion coating tends to be blackened by treatment with boiled water for sterilization. Further, the coating layer does not have satisfactory adhesion to a painted coating layer.
With respect to the coating solution disclosed in Japanese Patent Publication No. 56-33468, it shows sufficient properties when it is a fresh solution, namely a newly prepared solution. However, after repeated use for chemical treatment, aluminum is accumulated in the solution by etching of the aluminum plates or sheets with fluorine. A conversion coating produced by such a coating solution does not show high resistance to blackening by boiling water and good adhesion to a polymer coating film. In addition, the formed conversion coating does not have good slidability, so cans treated with this solution cannot smoothly be conveyed.
Further, the treatment solution disclosed in Japanese Patent Laid-Open No. 56-136978 needs a treatment at a relatively high temperature for a long period of time, preferably at 50.degree.-80.degree. C. for 3-5 minutes, and the formed conversion coating does not have sufficient resistance to blackening by boiling water and sufficient adhesion to a polymer coating film. In addition, since the formed conversion coating is grayish, it cannot be suitably applied to aluminum cans for drinks.